There is an increased demand with respect to the high capacity optical transmission systems in backbone optical transmission systems. Implementation of WDM (Wavelength Division Multiplexing) optical transmission system having a bit rate of 40 Gb/s has begun. As a technique for implementing such a 40-Gb/s WDM optical transmission system, a system newly constructed can be conceived in which all the channels have a bit rate of 40 Gb/s. However, such a system leads to high implementation costs. In order to solve such a problem, an implementation technique has been proposed in which a part of the channels included in an existing 10-Gb/s WDM optical transmission system is gradually replaced with the 40-Gb/s WDM channels. With such an arrangement, the WDM optical transmission system may provide a 10-Gb/s optical signal and a 40-Gb/s optical signal in a mixed manner.
WDM optical transmission system compensates chromatic dispersion that occurs in an optical transmission path. It is known that the optimum value for the dispersion compensation changes according to the bit rate of an optical signal and a modulation format. Accordingly, such an arrangement, in which optical signals that provide bit rates that differ from one another are handled in a mixed manner as described above, has a problem regarding how the suitable dispersion compensation is to be determined.
Relating to the aforementioned problem, dispersion compensation techniques have been proposed in Japanese Laid-open Patent Publication No. 2005-065019 and Japanese Laid-open Patent Publication No. 2005-295126. The systems disclosed in the Japanese Laid-open Patent Publication No. 2005-065019 and the Japanese Laid-open Patent Publication No. 2005-295126 are optical transmission systems including a compensation node for each predetermined span, in addition to a in-line repeater employing a dispersion compensation fiber (DCF) or the like. At the compensation node, the WDM signals are optically demultiplexed, and are subjected to dispersion compensation in increments of bit rates. Subsequently, optical multiplexing is performed, and the WDM signals thus optically multiplexed are transmitted to the optical transmission path.
In the WDM optical transmission systems disclosed in the aforementioned the Japanese Laid-open Patent Publication No. 2005-065019 and the Japanese Laid-open Patent Publication No. 2005-295126, at a compensation node having a function as the optical add drop multiplexer (OADM) or a function as the optical cross connect (OXC), optical signals multiplexed in the form of WDM signals are separated in increments of bit rates, and dispersion compensation is performed for each optical signal. On the other hand, the dispersion compensation amount provided by the in-line repeater is set to a predetermined common value for all the bit rates, giving consideration to residual dispersion. That is to say, the in-line dispersion compensation amount set for the existing 10-Gb/s WDM signals are employed without change for the WDM signals including the 10-Gb/s signals and the 40-Gb/s signals in a mixed manner.
In a case in which the in-line dispersion compensation is adjusted for one of the optical signals having a particular bit rate as described above, the compensation error in the in-line repeater may be accumulated at each compensation node, leading to marked deterioration of the waveform. This leads to a problem of reduction in the WDM signal transmission distance. For example, in a case in which the transmission distance has been reduced, there is a detriment to increase the number of compensation nodes, which are expensive. That is to say, optimum in-line dispersion compensation for optical signals having a particular bit rate is not employed suitably for other optical signals having different bit rates. In some cases, such in-line dispersion compensation may reduce the performance of the WDM optical transmission system.